Touch-responsive circuit in electronic musical instrument

ABSTRACT

In a touch-responsive circuit adapted for use in an electronic musical instrument provided with a keyboard, and comprising a key depression speed detector including a change-over switch, a capacitor for storing electric charge and a discharging resistor, and a controlling signal generating means including a gating transistor controlled by the key depression speed detector and another capacitor for storing electric charge through said transistor, a voltage-compensating element is connected in the key depression speed detector so as to compensate for the threshold voltage of the gating transistor thereby raising the minimum output voltage of the key depression speed detector and insuring the sound generation for the pianissimo performance.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

This invention relates to tone level control in an electronic musical instrument, and more particularly, it pertains to touch-responsive control of the tone signal level in an electronic musical instrument.

(B) Description of the Prior Art

In an electronic musical instrument provided with a keyboard, the tone pitch is determined by the selection of the keys on the keyboard. Here, the tone level desirably is controlled by the speed of the depression of the desired key. Namely, it is desirable that, when a key is depressed strongly or at a high depression speed, a strong tone or a high level signal is generated, whereas when a key is depressed softly, a weak tone or a low level signal is generated. The correspondence between the key depression and the tone generation should desirably be held in the whole dynamic range.

In the conventional touch-responsive circuit desired for use in an electronic musical instrument, however, there exists a lower limit in the key depression for generating a tone. That is, in an extreme pianissimo performance, it may happen that no sound is generated from the instrument.

FIG. 1 shows an example of the conventional touch-responsive circuit designed for use in an electronic musical instrument provided with a keyboard. The circuit of FIG. 1 corresponds to one key and similar touch-responsive circuits are provided for the respective keys in the keyboard. In this figure, a tone generator TG generates a tone signal of a predetermined tone pitch corresponding to the key and a gate circuit GC (usually called keyer) controls, by a control signal, the signal level or the envelope of the tone signal to be delivered therefrom. The circuit for generating this control signal, i.e. the touch-responsive circuit, basically consists of a pair of charging and discharging circuits wherein the charging voltage for the pair's second circuit is controlled by the discharging voltage of the pair's first circuit and represents the speed of the key depression, i.e. the touch, and the voltage across a capacitor of the second circuit is fed to the gate circuit as the control signal.

A change-over key switch SW₁ is interlocked with the key of the keyboard and has: one stationary (normally closed) contact connected to a dc voltage source E₁, another stationary (normally open) contact connected to the base of a transistor Q₁, and a movable contact A connected to a parallel connection of a capacitor C₁ and a resistor R₁. The dc voltage source E₁, the change-over switch SW₁ and the parallel connection of the capacitor C₁ and the resistor R₁ connected to the movable contact of the switch SW₁ jointly constitute a key depression speed detector. More specifically, when the key is in its released state, the movable contact A is connected to the dc voltage source E₁ as shown in FIG. 1 and the capacitor C₁ is charged up to the source voltage E₁. When the key is depressed, the movable contact A is turned over to the base of the transistor Q₁. It should be understood that, during this part of operation, since the resistor R₁ forms a discharging circuit for the capacitor C₁, the voltage V_(cl) across the capacitor C₁ decreases with time and hence the initial voltage applied to the base of the transistor Q₁ depends on how quickly the connection of the movable contact interlock with the key is changed over to the base of the transistor Q₁, i.e. the initial voltage depends on the key depression (movable contact travelling) speed. A base resistor R₂ is connected between the base of the transistor Q₁ and the ground. The collector-emitter circuit of the transistor Q₁ forms an electronic switch of a charging circuit for a capacitor C₂ from a voltage source terminal V_(cc) through a resistor R₄. The resistor R₄ serves to determine the charging time constant for the capacitor C₂. As the resistor R₄ is of a very small resistance value, the capacitor C₂ is charged up rapidly. More particularly, when the movable contact A of the change-over switch SW₁ is connected to the base of the transistor Q₁, the voltage V_(cl) across the capacitor C₁ is applied to the base of the transistor Q₁ and the transistor Q₁ is turned "on" to an extent corresponding to the base potential. Then, the capacitor C₂ is charged up to a voltage determined by the base potential of the transistor Q₁ which represents the key depression speed. The voltage V_(cl) across the capacitor C₁ decreases due to the discharge through the resistors R₁ and R₂ (where R₂ >> R₁), and consequently the base potential of the transistor Q₁ falls to the ground level so that the transistor Q₁ is turned off. Thus the transistor Q₁ and the capacitor C₂ constitute a peak hold circuit of the voltage appearing at the movable contact A. Then, the charge stored in the capacitor C₂ begins to discharge through the resistors R₃ and R₄. Resistors R₃ and R₄ determine the discharging time constant for the capacitor C₂. Such a decaying voltage V_(c2) is applied to the gate circuit GC for determining the envelope of the tone signal. Thus, the gate circuit GC allows the passage of a tone source signal having a level or an envelope corresponding to the key depression speed, i.e. the touch.

In such a touch-responsive circuit, however, it should be understood that, when a key is depressed very slowly, the charge stored in the capacitor C₁ may be discharged out substantially during the period that the movable contact A travels from the dc voltage source E₁ to the base of the transistor Q₁. Namely, when the key is depressed and the movable contact A is disconnected from the dc voltage source E₁, the voltage V_(cl) across the capacitor C₁ begins to decrease as shown by the broken curve in FIG. 3. In FIG. 3, the ordinate represents the voltage V_(cl) across the capacitor C₁ (potential at the movable contact) and the abscissa represents "time" from the disconnection of the movable contact A off the dc voltage source E₁ (i.e. key depression). Here, since the resistance of the base circuit of the transistor Q₁ may be substantially large as compared with the resistance R₁, the connection of the movable contact A to the base of the transistor may not alter the curve substantially. When the potential at the movable contact decreases below some level before the movable contact somes into contact with the base of the transistor Q₁, the transistor Q₁ does not become sufficiently conductive to charge the capacitor C₂ to a level sufficiently high to open the gate circuit GC. Then, no tone signal is derived from the tone generator TG through the gate circuit GC even when the key is depressed.

More particularly, in the circuit of FIG. 1, let us now assume that the threshold base-to-emitter voltage of the transistor is V_(th) (>0), the voltage V_(cl) at the movable contact A of the switch SW₁ should not decrease below the threshold voltage V_(th) to insure tone production. Since, however, the voltage V_(cl) is arranged to drop from E₁ toward zero through the resistor R₁, treating the threshold voltage V_(th) only as an arbitrary transient (passing point) voltage, a pianissimo performance may not generate a controlling voltage above the threshold voltage, and hence any sound will not be produced.

SUMMARY OF THE INVENTION

An object of the present invention, therefore, is to provide a touch-responsive circuit for controlling a tone signal level by the touch of a key operation, adapted for use in an electronic musical instrument provided with a keyboard and being capable of insuring the weakest tone generation corresponding to the pianissimo performance.

Another object of the present invention is to provide a touch-responsive circuit for controlling a tone signal level by the depression of a key operation on the keyboard, adapted for use in an electronic musical instrument provided with a keyboard and comprising a transistor having a threshold voltage and a discharging circuit connected to the transistor and including a voltage-compensating element for generating a voltage at least equal to the threshold voltage of the transistor.

According to an aspect of the present invention, there is provided a touch-responsive circuit adapted for use in an electronic musical instrument provided with a keyboard, comprising: a voltage-controlled circuit means having a threshold voltage for generating an output signal, and a key depression speed detector for generating a voltage signal having a level corresponding to the depression speed of a key in the keyboard and at least equal to the threshold voltage for controlling the voltage-controlled circuit means.

According to the above aspect, the controlling voltage signal never decreases below a certain level, and hence the tone generation is assured even for the pianissimo performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an example of the conventional touch-responsive circuit for use in an electronic musical instrument.

FIG. 2 is a circuit diagram representing an example of the touch-responsive circuit for use in an electronic musical instrument according to the present invention.

FIG. 3 is a chart of the capacitor voltage vs. time in the key depression speed detecting circuit for the comparison between the conventional and the present touch-responsive circuits.

FIG. 4 is a chart of the output voltage shapes of the touch-responsive circuit shown in FIG. 2.

FIG. 5 is a circuit diagram of another example of the touch-responsive circuit according to the present invention.

FIG. 6 is a circuit diagram of still another example of the touch-responsive circuit according to the present invention.

FIG. 7 is a circuit diagram of yet another example of the touch-responsive circuit according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The conventional touch-responsive circuit as shown in FIG. 1 has a drawback that a pianissimo performance may not generate any sound. This is because the output level of the key depression speed detector can drop below a certain threshold value for generating a sound.

This can be prevented by compensating for the threshold voltage by some means or other. One of the most appropriate method is to insert an compensating element in the key depression speed detector for insuring the threshold voltage. Generally, a transistor is used as the gating element for charging a capacitor, and the output of a key depression speed detector controls this transistor as shown in FIG. 1. The voltage drop between the base and the emitter in the transistor constitutes a threshold voltage for generating tone. For compensating for this threshold voltage, the use of a similar pn-junction (i.e. diode) is very effective.

An example of use of a diode for compensating for the threshold voltage of a transistor is shown in FIG. 2. Throughout the figures, similar reference letters denote similar parts.

In FIG. 2, a key depression speed detector includes a series connection of a resistor R₁ and a diode D₁ in place of the resistor R₁ shown in FIG. 1. That is, the discharging circuit for a capacitor C₁ is formed of the resistor R₁ and the diode D₁ which provides a certain forward voltage. Thus, the voltage V_(c1) across the capacitor C₁ decreases toward that voltage as shown by the solid curve in FIG. 3. In FIG. 3, letter V_(D1) represents the minimum forward voltage drop V_(D1) in the diode D₁ which is substantially equal to the threshold value of the transistor Q₁.

When the key is depressed, the interlocked contact A of the switch SW₁ is disconnected from the dc voltage source E₁, and the voltage V_(c1) across the capacitor C₁ begins to decrease. Here, even if the key is depressed very slowly, i.e. when it takes a considerable time for the contact A to swing over from the voltage source E₁ to the base of the transistor Q₁, the voltage V_(c1) will never decrease below V_(D1). Therefore, when the contact A comes into contact with the base of the transistor Q₁, it supplies a voltage at least equal to the threshold voltage of the transistor Q₁, and the transistor Q₁ becomes conductive to a corresponding degree.

Thus, the transistor Q₁ for charging a capacitor C₂ from a dc voltage source V_(CC) through a resistor R₄ is controlled in a touch-responsive manner in the full range of the key depression. The voltage across the capacitor C₂ is applied to the gate circuit GC for controlling the passage of the tone signal from the tone generator TG. An emitter resistor R₃ for the transistor Q₁ also forms a discharging path for the capacitor C₂ jointly with the resistor R₄. A series connection of a resistor R₅ and a diode D₂ is connected between another dc voltage source E₂ and the capacitor C₂ to form another discharging path for the capacitor C₂. The voltage of E₂ is lower than that of V_(CC). A series connection of a resistor R₆, a diode D₃ and switches SW₂ and SW₃ is connected between the capacitor C₂ and the ground to form still another discharging path for the capacitor C₂. The diodes D₂ and D₃ are used for preventing the reverse current flow. The switch SW₂ is interlocked with the key similar to the switch SW₁ and is closed when this key is released, when the switch SW₁ is connected to the voltage source E₁. The switch SW₃ is normally closed and opened by the depression of a damper (or sustain) pedal. The resistor R₃ has a relatively large resistance as compared with those of the resistors R₅ and R₆. The resistor R₅ has a relatively small resistance value to expedite the discharge.

Next, the operation of the circuit discussed above will be described. When the key is depressed, a dc voltage dependent upon the key depression is applied to the base of the transistor Q₁ and the capacitor C₂ is charged up rapidly to that dc voltage through the transistor Q₁ and the resistor R₄ as the resistor R₄ has a very small resistance. The charged-up voltage across the capacitor C₂ opens the gate circuit GC to an extent determined by that voltage. At this point, the switch SW₂ is in the open state. Then, the capacitor C₂ discharges mainly through the diode D₂ and the resistor R₅ with a time constant of C₂ ·R₅. Here, since the resistance of the path including the resistors R₃ and R₄ is large, the effect of this path can virtually be neglected in this initial decaying period. Since the discharging path including the resistor R₅ and the diode D₂ is connected to the dc potential E₂, it is virtually cut off when the voltage V_(c2) across the capacitor C₂ approaches the potential E₂. Then, the discharging path including the resistors R₃ and R₄ plays the main role with a time constant C₂ (R₃ +R₄). When the damper switch SW₃ is kept closed (i.e. the damper pedal is not depressed) and the switch SW₂ returns back to the closed state, the discharging path through the resistor R₆ , the diode D₃ and the switches SW₂ and SW₃ is formed to expedite the discharge of the capacitor C₂. Then, the voltage V_(c2) across the capacitor C₂ decreases rapidly after the key release as shown by the solid curve in FIG. 4. If the damper pedal is released (i.e. the switch SW₃ is closed) after the release of the key, it will be apparent that the accelerated damping effect occurs thereafter.

More particularly, while the key is being depressed (the switch SW₂ is open), the discharging path through the resistor R₅ and the diode D₂ plays the main role in the initial stage and the discharging path through the resistors R₃ and R₄ acts in the succeeding stage. Since the resistance R₃ is large, the decaying in the succeeding stage is slow. When the key is released with the damper pedal being not depressed, the discharging path through the resistor R₆ and the diode D₃ becomes dominant to swiftly discharge the capacitor C₂. On the other hand, when the key is released with the damper pedal being depressed, the above-mentioned slow decay continues, as the R₆ -D₃ discharging path is not established.

A modification of the circuit of FIG. 2 is shown in FIG. 5, in which two diodes D₁₁ and D₁₂ are connected in series to the resistor R₁, and a short-circuiting switch SW₄ is connected in parallel to one D₁₁ of the two diodes in place of the single diode D₁ in the circuit of FIG. 2. When the switch SW₄ is closed, the circuit in this example is similar to the circuit of FIG. 2. When the switch SW₄ is open, however, the minimum level of the base bias voltage for the transistor Q₁ increases. Hence, the addition of a diode D₁₁ and a short-circuit switch SW₄ enables the choice of the minimum output level. It will be apparent that the number of the additional diodes is not limited to one.

In the circuit of FIGS. 2 and 5, the transistors Q₁ and the voltage source V_(CC) may be replaced by a diode for simplifying the circuit. In such a case, the capacitor C₂ is charged by the discharging current from the other capacitor C₁.

FIG. 6 shows another example of the touch-responsive circuit, in which the number of diodes in the key depression speed detector for all the keys is reduced to one. The touch-responsive circuitry for each key is shown by the portion sorrounded by a chain line. The key depression speed detector is similar to the conventional structure shown in FIG. 1. The only difference, however, is found in the arrangement that the parallel connection of the capacitor C_(1a) and the resistor R_(1a) is connected to the ground potential not directly but through a common diode D₁. Comparing this example with that of FIG. 2, this example has the advantage that the number of the diodes employed in the circuitry can be greatly reduced. Here, it should be noted that when a key is depressed and the connection of the corresponding switch e.g. SW_(1a) changed over to the base of the corresponding transistor Q_(1a), the diode D₁ remains to be still connected to the dc potential source E₁ through the resistor R_(1b) to R_(1n) in the other key depression speed detectors, and thus generates a voltage drop V_(D1) thereacross.

FIG. 7 shows an alternation of the example of FIG. 6, in which the selection of the minimum level is enabled similar to the circuit of FIG. 5 by the addition of a diode (or diodes) and a short-circuiting switch (or switches). The operation of this circuit will be apparent from the description of the circuits of FIGS. 5 and 6.

It will be seen that the diode D₁ in the circuits of FIGS. 6 and 7 may be replaced by a resistor. 

We claim:
 1. A touch-responsive circuit adapted for use in an electronic musical instrument provided with a keyboard comprising:a voltage-controlled circuit means having a threshold voltage for generating an output signal; and a key depression speed detector for generating a voltage signal having a level corresponding to the depression speed of a key in the keyboard, said key depression speed detector including a first constant dc voltage source, a first capacitor for storing an electric charge from said first dc voltage source and a discharging circuit means for discharging said first capacitor including a voltage-compensating means comprising a first semiconductor element for generating a voltage drop thereacross at least equal to said threshold voltage.
 2. A touch-responsive circuit according to claim 1 in which:said voltage-controlled circuit means includes a second semiconductor element having said threshold voltage for allowing a current flow.
 3. A touch-responsive circuit for use in an electronic musical instrument provided with a keyboard having a multiplicity of keys, comprising:a common voltage-compensating means; and a multiplicity of circuit units, each being designated to one key, each of said circuit units comprising a voltage-controlled circuit means having a threshold voltage for generating an output signal, and a key depression speed detector for generating a voltage signal of a level representing the depression speed of the designated key, each of said key depression speed detectors including a parallel connection of a capacitor and a resistor connected to a reference potential through said common voltage-compensating means, said common voltage-compensating means generating a voltage at least equal to said threshold voltage and being added to the voltage signal of said key depression speed detector to provide a voltage signal for controlling said voltage-controlled circuit means.
 4. A touch-responsive circuit according to claim 2, in which:said first and second semiconductor elements are diodes.
 5. A touch-responsive circuit according to claim 1, in which:said discharging circuit means further includes a parallel connection of a diode and a short-circuiting switch connected in series to said first-semiconductor element.
 6. A touch-responsive circuit according to claim 2, in which:said voltage-controlled circuit means further includes a second capacitor to be charged through said second semiconductor element, a second dc voltage source, a first discharging path consisting of resistive means and connected between said second capacitor and the ground, and a second discharging path including a diode for preventing the reverse current-flow and connected between said second capacitor and said second dc voltage source.
 7. A touch-responsive circuit according to claim 6, in which:said voltage-controlled circuit means further includes a third discharging path including a damping switch for expediting the discharge of said second capacitor when the damping switch is closed.
 8. A touch-responsive circuit according to claim 7, in which:said third discharging path includes another switch interlocked with a corresponding key in the keyboard and being closed when the key is in the released position so that an enhancement of the discharge of said second capacitor takes place only when the key has returned to the released position.
 9. A touch-responsive circuit according to claim 3, in which:said common voltage-compensating means is a semiconductor element.
 10. A touch-responsive circuit according to claim 2, in which:said first and second semiconductor elements are a diode and a transistor, respectively. 